Electroless copper plating baths are used in metallization industries for depositing copper on various types of substrates. For example, in the manufacture of printed wiring boards the electroless copper baths are used to deposit copper into through-holes and circuit paths as a base for subsequent electrolytic copper plating. Electroless copper plating is also used in the decorative plastics industry for deposition of copper onto non-conductive surfaces as a base for further plating of copper, nickel, gold, silver and other metals. Typical baths which are in commercial use today contain divalent copper compounds, chelating agents or complexing agents for the divalent copper ions, formaldehyde reducing agents and various addition agents to make the bath more stable, adjust the plating rate and brighten the copper deposit. Although many of such baths are successful and are widely used, the metallization industry has been searching for alternative electroless copper plating baths that do not contain formaldehyde due to its toxic nature.
Formaldehyde is known as an eye, nose and upper respiratory tract irritant. Animal studies have shown that formaldehyde is an in vitro mutagen. According to a WATCH committee report (WATCH/2005/06—Working group on Action to Control Chemicals—sub committee with UK Health and Safety Commission) over fifty epidemiological studies have been conducted prior to 2000 suggested a link between formaldehyde and nasopharyngeal/nasal cancer but were not conclusive. However, more recent studies conducted by IARC (International Agency for Research on Cancer) in the U.S.A. showed that there was sufficient epidemiological evidence that formaldehyde causes nasopharyngeal cancer in humans. As a result the INRS, a French agency, has submitted a proposal to the European Community Classification and Labelling Work Group to reclassify formaldehyde from a category 3 to a category 1 carcinogen. This would make usage and handling of it more restricted, including in electroless copper formulations. Accordingly, there is a need in the metallization industry for a comparable or improved reducing agent to replace formaldehyde. Such a reducing agent must be compatible with existing electroless copper processes; provide desired capability and reliability and meet cost targets.
Hypophosphites have been used as a replacement for formaldehyde, such as in U.S. Pat. No. 4,265,943; U.S. Pat. No. 5,562,760; U.S. Pat. No. 6,046,107; and U.S. Pat. No. 6,534,117; however, hypophosphites do not possess catalytic function on the copper surface. This problem is addressed by the addition of extra mediator metal ions, such as nickel(II) and cobalt(II), to the plating bath. However, the copper deposit from such baths is not sufficiently conductive for many electronic applications.
Glyoxylic acid is another reducing agent which has been used to replace formaldehyde. Glyoxylic acid is similar in structure to formaldehyde but its oxidation potential is more positive than formaldehyde. An example of a copper bath which includes glyoxylic acid as a reducing agent is U.S. Pat. No. 7,473,307. However, glyoxylic acid is much more expensive than formaldehyde and it decomposes quickly. Glyoxylic acid requires a strong alkaline environment to function as a reducing agent because of its acidic nature. Readily obtainable and conventional alkaline compounds, such as sodium hydroxide are not suitable for adjusting the pH due to the formation of insoluble sodium oxalate as a by-product. Such a by-product shortens the lifespan of the plating bath. Accordingly, the more expensive base potassium hydroxide is typically used to adjust the alkalinity to the desired pH of at least 12. All of the above features discourage the use of glyoxylic acid as a reducing agent for electroless copper baths.
Boron containing compounds such as dimethylamine borane, sodium borohydride and ammonium tetra(n)-butylboranhydride have also been tried as reducing agents with various degrees of success. Examples are disclosed in U.S. Pat. No. 4,684,550; and U.S. Pat. No. 4,818,286. Unfortunately, these reducing agents are very expensive and associated with health and safety hazards. Resultant borates have adverse effects on crops and dimethylamine borane is generally toxic.
Accordingly, there is still a need for an electroless copper bath which is free of formaldehyde and is both stable, provides acceptable copper deposits and is environmentally friendly.